How to make div stick to the bottom of parent div?

.wrapper{position:relative;}
.bottom{position:absolute; bottom:0;}

Edited

Thanks to @centr0 and @T.J Crowder
the key thing to understand here is that position:relative in .wrapper will “contain” any element that has position:absolute. It’s like declaring boundaries for .bottom. Without position:relative in .wrapper, .bottom would break out of that div